The invention relates generally to a variable stroke positive displacement liquid metering pump for use in a printing couple where it is desirous to accurately regulate the amount of ink being metered. Various type ink pumps have been proposed for use in printing couples, as for example pumps as disclosed in U.S. Pat. Nos. 1,311,198 and 1,348,198 which utilize plungers which are moveable in reciprocal directions in bores in combination with moveable slides having liquid passageways therein. Further examples of metering pump constructions which utilize plungers moveable in bores are shown in U.S. Pat. Nos. 1,185,667 and 1,214,856 but which require use of separate valves in the form of ball checks. Such pumps have been subjected to problems in that they are not easily adjusted in order to vary the amount of ink being pumped or that they have utilized moveable parts such as ball checks and slides which, because of the nature of the liquid being pumped, often are subjected to undue wear resulting in inaccurate metering. This is in part because printing ink is not a true liquid but rather a suspension of finely dispersed solids in an oily carrier fluid where the solids act as an abrasive. A further difficulty arising from using pumps having valves is the difficulty of accurately metering flow due to inherent non-lineal flow characteristics of valves. Also use of valves increases problems of leaking and "bleeding" which becomes of particular concern when the amount of liquid to be metered is small or approaches zero.
Valveless pump constructions have been proposed which utilize plungers moveable in pump casings where the plungers have a cut-out portion and means are included to rotate the plunger in timed relationship to its longitudinal movement, such that the cut-out portions overlie outlet and inlet ports. For example, see the constructions of U.S. Pat. No. 3,636,873 and U.S. Pat. No. 3,914,073. Such constructions however have required use of complicated and expensive means to vary the stroke of the plungers in order to vary metering. It is therefore an object of my invention to provide for a pumping unit which has the advantages of a valveless construction and which is adapted for accurately metering an abrasive fluid such as ink while at the same time providing easily manufactured and dependable structure for varying the metering by varying the intake stroke of a pump plunger.